During the refining of comminuted cellulosic material (e.g., wood chips) to produce mechanical pulps, some of the fibers of the pulp become distorted (e.g., twisted, kinked, or curled). The pulp is then said to have "latent properties" and unless the latency is removed, it can be difficult to effectively screen the pulp and/or it is difficult to produce from the pulp paper products having desired properties.
In typical commercial practice, latency removal is effected by passing the pulp to a latency chest. In the latency chest, pulp is agitated at a consistency of about 1.25-2% in a temperature range generally between about 70.degree. C.-90.degree. C., for twenty or thirty minutes or more. after agitation for that period of time, the pulp is pumped to a screening room.
In another prior art proposal, as disclosed in U.S. Pat. No. 4,361,464, a predetermined sample of pulp is placed in a container, and then is pumped in a circulatory path from the container, and back to the container, by a conventional centrifugal pump. Recirculation is effected at a rate of at least five times a minute, and utilizing such a technique it is possible to effect substantially complete latency removal in three minutes, and perhaps in as little as one minute.
According to the method and apparatus of the present invention, latency removal is accomplished with a speed and simplicity vastly superior to those in the prior art. According to the present invention, latency removal can be effected without introducing any delay whatsoever in the treatment of the pulp, and without the necessity for a separate tank, agitator, or the like. Such a remarkable result is achieved according to the present invention by effecting fluidization of the pulp to effect latency removal, the fluidization being effected during pumping of the pulp from one stage to another utilizing a centrifugal pump capable of pumping pulp at a consistency of between about 8-25%, and ideally between about 12-25%. Several forms that such a pump can take are shown in U.S. Pat. Nos. 4,435,193 and 4,410,337, and Canadian Pat. No. 1,102,604.
In the practice of the present invention, it is possible to effect latency removal merely by pumping the pulp in a non-return path from a dilution stage to a screening stage. Refining typically takes place at a consistency of 25-45%, and the refined pulp is diluted to a consistency of about 8-25%, and then pumped to the screening stage. The pulp may be stored in a storage tank after pumping and just before the screening stage, and if desired in order to enhance latency removal another pump may be provided between the storage stage and the screening stage. Also where desired a portion, and only a portion, of the pumped pulp may be recirculated back to the pump inlet.
It is the primary object of the present invention to provide a method and apparatus to effect quick and simple mechanical pulp latency removal. This and other objects of the present invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.